Most of today's metropolitan area network (MAN) infrastructure is based on the Synchronous Optical NETwork (SONET) adopted by the American National Standards Institute (ANSI) as a standard for fiber optic networks. SONET uses one optical fiber to transmit all data traffic and maintains a second optical fiber on standby. Should the working optical fiber fail, SONET automatically detects the failure and moves the data traffic to the standby optical fiber.
An alternative to using SONET as the provisioning platform for networking service is to use a bridged network like the Ethernet. A problem with bus and ring networks like the Ethernet is the possibility of a single point of failure in the network. A common solution is to design the network with redundant segments and loops so that there is more than one route between nodes in the network. Redundancy and loops can, however, present another problem in which transmission of a broadcast packet or an unknown unicast packet results in a broadcast storm where each node receives and rebroadcasts the packet causing potentially severe network congestion.
One way known in the industry of preventing broadcast storms and other unwanted side effects of looping is to use the Spanning Tree Protocol (STP), based on a spanning tree algorithm that has been standardized in the 802.1D specification by the Institute of Electrical and Electronic Engineers (IEEE Std. 802.1D-1998, IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Common specifications). With the STP, bridges or switches in the network dynamically calculate an optimum subset of the network topology that is loop-free yet allows a single, primary, path to each node in the network. Alternate paths are blocked but can be unblocked as necessary to keep communication open when a fault occurs in a primary path selected by the STP. A significant problem with the STP is that when a link breaks, it may take a significant period of time, e.g., thirty or more seconds, for an alternate path around the problem to be calculated and traffic successfully rerouted. This level of performance is too slow for use in today's local area networks (LANs) and metropolitan area networks (MANs).
An alternative to using Ethernet with STP is described in U.S. pending patent application Ser. No. 09/999,796, filed on Oct. 31, 2001, entitled Ethernet Automatic Protection Switching, and assigned to the assignee of this invention. The pending patent application discloses a method in which an Ethernet automatic protection switching (EAPS) system prevents loops in a layer-2 network having a ring topology.
The EAPS system provides for one or more EAPS domain on a single Ethernet ring. An EAPS domain is configured on the physical ring. Nodes, such as bridges, switches, other packet-forwarding devices, network server computers, end stations, or host computers, are connected to the ring. For each EAPS domain, there is designated a master node. All other nodes on the ring are designated transit nodes. The master node may be configured at the time of network installation and set-up. On the master node, one port is designated as the primary port, and another port is designated as a secondary port. In normal operation, the master node blocks the secondary port from transmitting or receiving data traffic to prevent a loop in the ring. This makes it possible to deploy and use standard Ethernet switching and learning algorithms on the ring network topology. If the master node detects a ring fault, it unblocks its secondary port and allows frames of Ethernet data traffic to pass through the secondary port.
At least one virtual network, such as a virtual local area network (VLAN), that is to be protected by the EAPS domain, is configured on the ring as well. In particular, the virtual network is configured on each port of each node connected to the ring. The virtual network includes a control virtual network, for example, a control virtual local area network (VLAN), and at least one data virtual network, for example, a data VLAN. Control messages are transmitted over the control VLAN and pass through all ports of all nodes, including the secondary port of the master node.
The master node detects a network failure by means of these control messages sent between the master node and the transit nodes using the control VLAN. As stated above, during normal operation, the master node blocks the data traffic on the data VLAN from traversing its secondary port. However, during a network failure, the master node reroutes the data traffic on the data VLAN through its secondary port. When the network is restored and again capable of normal operation, the EAPS system prevents data traffic looping through the network by blocking the data traffic on the data VLAN until the master node notifies the transit nodes that the normal operation has resumed and blocks its secondary port.
A problem can occur when a single virtual network spans multiple rings. Each ring is associated with a separate EAPS domain, and may be connected together via a segment of its ring that is shared with the other ring, such as link 3 in FIG. 1. A segment may comprise one or more links and nodes between two nodes. When there is a failure in a shared segment, the master node in each respective ring unblocks its secondary port, thereby creating a loop that spans both rings in the virtual network. One approach to addressing this problem is to employ the spanning tree protocol (STP) to block a segment and thereby stop data traffic looping through the multi-ring network, but configuring both STP and EAPS complicates both configuring and managing the network. Additionally, the STP is slow to converge to a new network topology in the event of a network failure, compared to EAPS.